Conditional lethal L cells were created by transfer of a temperature-sensitive thymidine kinase gene from Herpes Simplex Virus Type 1. At 39 degrees C and in low thymidine 1 microgram/ml, the cells are killed. Increasing the level of thymidine or decreasing the temperature will result in rescue of the cells. The TKts gene will be used as a selective vector for gene transfer and as a means of using temperature to increase the gene copy number in the cell.